Why do I need a much bigger board AND bigger kite for a heavier rider?

[Matteo V]

Sure, absolutely agreed. No one can find all the variables. A bit like big models (like climate models for example).
Put it this way - if I put on 20 kg of lead weight on the same 75 kg boarder - what should increase in board and kite size be (the same kite and board make/variant)? Roughly as an educated guess?

Roughly as an educated guess - go with the next kite size up, or go with a bigger board. If you put 30kg of lead on them, then go the next size up board and kite.

The second biggest problem you face is that your kite quiver is likely spaced in 2m increments of flat area. And you really need to understand what "flat area" is and it's relationship to "projected" area. So............you just go up a size and ride it. Boards are extremely complicated from a mathematical point of view. And what you need to understand about a "bigger board" is that you apply rail pressure from the same lever arm (your foot size) in all board sizes/widths. So essentially, with a slightly larger board (width is most important), you have to use a different equation. Or you just ride the board a bit different and then any comparison goes right out the window.

The biggest problem you have, and please let me say I do not intend to insult you with this (I am serious), is that your question is not tackling a problem facing kitesurfers on modern kites.

Stupid question?

And no, this is not a stupid question. It is one of the best on this forum in a while. Many kiters wonder about this same thing at some stage in their kite skill development. This is a great question that sparked debate and yielded an answer that you may not have wanted, but that answer is simply the reality of the situation.

[Mike101]

I'd say you have to consider the lightest and heaviest wind days you enjoy riding in and then ask whether you'd rather a big board and a little board to enjoy these two extemes.

If your only going to go out when it's 16 to 30knts then one middle size kiteboard would probably do you. If you want to be able to ride 12 to 45knts like I do then 2 boards is better. Unless this is the wife posting in which case 7 boards is the bare minimum you need once you are past beginner.

[sergei Scotland]

Thanks Matteo - no offence taken.
On the other hand most engineering problems (probably 90%) are like this. It can be a plane, a rocket, an engine, a kite or a board. Noone can calculate exactly how it will behave and what performance will be using formulae.
In most cases it is empirical. For a plane or a kite one could use a wind-tube to estimate what a change does.
So how the hell engineers manage to build something that flies/drives etc.?
I think they do what I am trying to do - they try to figure out rough formulas first based on physics or if they can't - based on sets of empirical data.
Then they tweak until they get something good. Then a pilot flies a plane and they might change something based on pilot input etc.

My point is - although we can't calculate exact numbers (almost never - may be in designing a building it is more often possible) - we should always try to built rough models so we know roughly what to tweak when and roughly by how much.
The models can be just based on sets of empirical data - like "go next size up" - which is one way of doing it -
basically my question was a "request for empirical data"

[sergei Scotland]

Thanks Mike101.
As I say I am 95 kg (215 pounds). I have a 136 x 45 board which I understand is a bit small (I think my main board should be around 142x45 - correct?).
I have an old-ish 13m SLE.

So you (Mike) has just confirmed for me that I should be OK.
I should be using my 13 m when 75kg people use a 10 (providing it is safe etc). I do not really need bigger board that bad right now.

I.e. I have 26% increase in weight. Hopefully a 30% increase in kite size should be enough to compensate for it (providing my kite is not complete rubbish - I hope not).
Correct?

Obviously when 75 kg people are using 12m - I will be struggling with my board and 13 m

[grigorib]

Thanks Mike101.
As I say I am 95 kg (215 pounds). I have a 136 x 45 board which I understand is a bit small (I think my main board should be around 142x45 - correct?).
I have an old-ish 13m SLE.

So you (Mike) has just confirmed for me that I should be OK.
I should be using my 13 m when 75kg people use a 10 (providing it is safe etc). I do not really need bigger board that bad right now.

I.e. I have 26% increase in weight. Hopefully a 30% increase in kite size should be enough to compensate for it (providing my kite is not complete rubbish - I hope not).
Correct?

Obviously when 75 kg people are using 12m - I will be struggling with my board and 13 m

I'd say your board size is about perfect. Getting a 142 will improve lower end but would hinder the agility for freestyle and transportability.
13m for your weight should get you riding alright between 11-20knots and if it's a less efficient kite you can get a set of 10m extension lines to significantly improve lowend.

Set the math aside. Experience and gear matter a lot. Conditions matter a lot. And kites have their limitations for wind range on their own, completely abstracted from rider weight. And obviously people have a quiver, not one single kite so you cover the wind range at any time.

I'm bout 90kg. A friend of mine is about 75kg. Last year we rode in 16 knots side by side - he on his 12m Ozone Catalyst and I on 9m Turbine. I had way more fun. We swapped kites and he had way more fun than on his 12m. However his Catalyst has better drift and is a better downwinder/wave/foil kite than my Turbine. And the Turbine boosts to the skies.

To your last point - 75kg people shouldn't be using 12m. That's why it's hard to match the equation. They should be using 10m as their regular or 13m as their LW kite. At your weight you should be using 12m as your regular and 15m as your lightwind kite.
BUT! 75kg person should not be riding 45cm wide board unless for LW. They should be on 40cm or so which compensates greately the weight difference besides kite size.

[sergei Scotland]

Thanks grigorlib. Appreciated. Actually I did not realise 13m should give me that much low end with my board. I only expected to be able to ride from 15 knots really.... I guess I just need to get better!

[stakas]

This should be pretty simple from a physics point. The resistance comes from the wet area of the kiteboard.
It is roughly linear with your weight. Twice as heavy rider should use twice as big board and kite for surface area.

[Regis-de-giens]

So how the hell engineers manage to build something that flies/drives etc.?
I think they do what I am trying to do - they try to figure out rough formulas first based on physics or if they can't - based on sets of empirical data.
Then they tweak until they get something good. Then a pilot flies a plane and they might change something based on pilot input etc.

My point is - although we can't calculate exact numbers (almost never - may be in designing a building it is more often possible) - we should always try to built rough models so we know roughly what to tweak when and roughly by how much.
The models can be just based on sets of empirical data - like "go next size up" - which is one way of doing it

I fully agree on this approach. But people not comfortable with math have to think another way ... so not easy to convince that math can help in the understanding and preliminary estimations ...

Ok my view about theoretical impact of weight on sizes :

1- TT or surf drag is highly unlinear and not depending only of wet area (but board design, pressure, angle, speed, ect)

2- Hydrofoil is far more linear (the symetrical behaviour of the kite in the air, and no contact with unlinear water surface) , so we can use it as a better model base IMO ;

... so for hydrofoil now and in first approximation (open to remarks/corrections of those still awae at the end of this post) ; lets consider the low end for planning upwind limit as a school case, planning straight and kite parked stable in the window.

- a 200 lbs rider will need a wing that is twice larger to foil-up (HF linear with lift thus with weight )

- a twice larger wing (in area) will induce twice higher drag (linear considering the same foil wing design with identical L/D ratio).

- to pull twice higher drag you need twice larger kite (kite lift is linear with kite surface)

So ??? A double-burger rider needs to double the board AND the kite vs a little child. And double the line lenght but that is out of topic and explanation would start to be boring right ?

Now in real world we do not need to go to this extreme in my experience, specially for twintype where unlinearities and waterstarts could kill the linear reasonning IMO.

[fernmanus]

You guys are overthinking it. I used to try to measure the wind speed and compare it to wind meter strength. I would waste time on the beach trying to figure out what size of kite to put up. I would query other riders and try to think my way to the bes5 kite size.

I had a friend who taught me to observe instead. Look at he water, the trees, flags, other kites in the air. Feel the strength of the wind. Use your senses and you will be able to pick the right kite size intuitively. It also doesn’t hurt to have a kite with a wide win$ range.

[Beardytello]

You guys are overthinking it. I used to try to measure the wind speed and compare it to wind meter strength. I would waste time on the beach trying to figure out what size of kite to put up. I would query other riders and try to think my way to the bes5 kite size.

I had a friend who taught me to observe instead. Look at he water, the trees, flags, other kites in the air. Feel the strength of the wind. Use your senses and you will be able to pick the right kite size intuitively. It also doesn’t hurt to have a kite with a wide win$ range.

I agree with this.

Although I don't begrudge the scientific analysis (without it we wouldn't have kites...or most other fun things)

But for me I know from shared experience and someone else's scientific formula that depending on how hard my beard is whipping over my shoulder I should be on my 9, 14 or 17...the more I do this the less I get it wrong.